Method and structure for cooling a dual chip module with one high power chip

ABSTRACT

Disclosed is a cooling structure which has individual spreaders or caps mounted on the chips. The thickness of the high power spreader or cap exceeds the thickness of the lower power spreaders to ensure that the high power spreader achieves the highest plane and mates to a heat sink with the smallest interface gap. The variable and higher gaps between the lower power spreaders and the heat sink base are accommodated by compressible thermal pad or grease materials.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to cooling structures forintegrated circuit chips and more particularly to an improved coolingstructure that concentrates the largest cooling capacity on the highestpower chips.

2. Description of the Related Art

Multi-chip electronic packages with high power microelectronic chips arebeing increasingly encountered in computer and other electronic systemswhere a common chip carrier, whether a ceramic or an organic laminate,has a central processing unit (CPU) accompanied by one or more memorychips. These CPU and memory chips often operate at different powerlevels and, therefore, have different cooling needs. However,conventional structures do not address such cooling needs sufficiently.Therefore, the present invention has been created to provide cooling forsuch a multi-chip package by concentrating the largest cooling capacityon the highest power chips.

SUMMARY OF THE INVENTION

The invention presents a cooling structure for an integrated circuitstructure that has multiple integrated circuit chips. In one embodiment,the cooling structure comprises heat spreaders and a heat dissipatingstructure. The lower side of each of the heat spreaders is connected tothe top of a corresponding integrated circuit chip through thermallyconductive interface materials. The upper side of the heat spreaders isconnected to a heat dissipating structure through thermally conductivematerials which are positioned in gaps between the upper sides of theheat spreaders and the bottom of the heat dissipating structure. Thesmallest of the gaps exists between the top of the heat spreader that isconnected to the integrated circuit chip that produces the most thermalenergy, relative to the other integrated circuit chips, and the bottomof the heat dissipating structure.

In another embodiment, the cooling structure comprises a cap connectedto the chip carrier and to the top of the integrated circuit chip thatproduces the most thermal energy, relative to the other integratedcircuit chips. This embodiment also includes a plurality of heatspreaders, wherein the lower side of each of these heat spreaders isconnected to the top of a corresponding integrated circuit chip (of theother integrated circuit chips). In this embodiment, the heatdissipating structure is connected to the upper sides of the heatspreaders and the cap through thermally conductive material. Thethermally conductive material are positioned in gaps, wherein the gapsexist between the tops of the heat spreaders and the bottom of heatdissipating structure, and between the upper side of the cap and thebottom of the heat dissipating structure. Here, the smallest of the gapsexists between the upper side of the cap and the bottom of the heatdissipating structure.

In a further embodiment, the cooling structure comprises a heat spreaderconnected to the integrated circuit chip that produces the most thermalenergy, relative to the other integrated circuit chips and a heatdissipating structure connected to the upper side of the heat spreaderand to the upper sides of the other integrated circuit chips. Thermallyconductive materials are positioned in gaps that exist between the upperside of the heat spreader and the bottom of the heat dissipatingstructure, and between the upper sides of the integrated circuit chipsand the bottom of the heat dissipating structure. Again, the smallest ofthe gaps exists between the upper side of the heat spreader and thebottom of the heat dissipating structure.

In an additional embodiment, the cooling structure comprises a heatdissipating structure connected to the upper sides of the integratedcircuit chips through a thermally conductive material, where the heatdissipating structure is shaped such that the smallest gap existsbetween the top of the integrated circuit chip that produces the mostthermal energy and the bottom of the heat dissipating structure. In thisembodiment, the heat dissipating structure has a protrusion positionedadjacent the integrated circuit chip that produces the most thermalenergy to allow the smallest of the gaps to exist between the top of theintegrated circuit chip that produces the most thermal energy and thebottom of the heat dissipating structure.

These, and other, aspects and objects of the present invention will bebetter appreciated and understood when considered in conjunction withthe following description and the accompanying drawings. It should beunderstood, however, that the following description, while indicatingpreferred embodiments of the present invention and numerous specificdetails thereof, is given by way of illustration and not of limitation.Many changes and modifications may be made within the scope of thepresent invention without departing from the spirit thereof, and theinvention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following detaileddescription with reference to the drawings, in which:

FIG. 1 is a sectioned view of a chip-carrier with several chips mountedthereon;

FIG. 2 is a cooling structure showing a common cap with a thermalcompound between the chips and the cap;

FIG. 3 is an enhanced cooling structure showing a customized piston forcontrolling the chip to piston gap on the high power chip:

FIG. 4 is an enhanced cooling structure showing a heat spreader mountedon the high power chip;

FIG. 5 shows that the back-sides of the chips can be ground to achieve acommon plane;

FIG. 6 is a cooling structure showing a planar common spreader attachedto all the chips;

FIG. 7 is a cooling structure where a stepped spreader is used toachieve the smallest chip to spreader gap on the high power chip;

FIG. 8 is a cooling structure where individual spreaders are attached tochips with the spreader on the high power chip achieving the highestplane;

FIG. 9 is a cooling structure where the spreaders in FIG. 8 are replacedby caps;

FIG. 10 is a cooling structure where the spreaders in FIG. 8 arereplaced by caps; and

FIG. 11 is a cooling structure where the heat sink is modified to form acavity in which the high power chip and spreader are enclosed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present invention and the various features and advantageous detailsthereof are explained more fully with reference to the nonlimitingembodiments that are illustrated in the accompanying drawings anddetailed in the following description. It should be noted that thefeatures illustrated in the drawings are not necessarily drawn to scale.Descriptions of well-known components and processing techniques areomitted so as to not unnecessarily obscure the present invention. Theexamples used herein are intended merely to facilitate an understandingof ways in which the invention may be practiced and to further enablethose of skill in the art to practice the invention. Accordingly, theexamples should not be construed as limiting the scope of the invention.

FIG. 1 shows a multi-chip package where several chips 3 are mounted,through C4 interconnections 2, on a common chip carrier 1 to reduce theinter-chip electrical wiring. The top surfaces of the different chipslie in different planes due to the variations in the thickness of thedifferent chips 3, the heights of the C4 interconnections 2, and camber(distortion) in the chip-carrier 1. The chip-carrier 1 attaches to aprinted circuit card (not shown) through LGA, CGA, BGA or otherinterconnection types 4.

A cooling structure 9, shown in FIG. 2, comprises a common high thermalconductivity cap 5 adhered to the chip carrier 1 through a sealantmaterial 7 which may be an elastomer, epoxy or a mechanical seal such asan o-ring. The different gaps between the various chips 3 and the cap 5accommodate a thermal compound 6 which may be a grease, adhesive, orphase change material. The heat sink 9 can be attached to the cap 5though a thermal interface material 8 to dissipate the heat to theambient.

One drawback of the structure in FIG. 2 is that the cooling is notoptimized for the highest power chip (e.g., chip 31) since the smallestchip 3 to cap 5 gap may not be necessarily achieved at the highest powerchip 31. With the limited thermal conductivity of the presentlyavailable thermal compounds 6, the cooling capacity of the structure inFIG. 2 cannot match the requirements of many multi-chip packages. Inaddition if a high thermal mass cap 5 is used, problems with attachingthe multi-chip package to the printed circuit card can be expected forCGA and BGA interconnection types which require high-temperatureprocessing to reflow their constituent solder materials.

To enhance the cooling efficiency, other structures described in FIGS. 3and 4 have been created. In FIG. 3, a piston 10 is integrated into thecap 5 and attached with a solder or other adherent material 30. The gapbetween the highest power chip 31 and piston 10 is minimized during theassembly process by reflowing the solder 30 or inserting the adherentmaterial 30 after compressing the piston 10 to achieve a desired gapbetween the chip 31 and piston 10. The top surface of the cap 5 ismachined to obtain a flat surface (to avoid having the piston 10 stickout of the cap 5 and avoid having a recess above the piston 10) afterassembly in order to interface with a heat sink 9.

In FIG. 4, a heat spreader 11 is attached to the highest power chip 31with a high-conductivity thermal interface compound 12. Another thermalcompound such as 6 can be inserted between the spreader 11 and the cap5. The cooling designs in FIGS. 3 and 4 add cost and assemblycomplexity. Further, such structures do not diminish the difficulty withwhich cards with CGA and BGA interconnections 4 are attached. Forexample, problems arise due to the high weight and thermal mass of thecapping structure in pick-and-place and solder reflows for cardattaching.

FIG. 5 illustrates another cooling structure that is produced bygrinding the back-side of the chips 3, 31 to achieve a common plane 13as shown in FIG. 5. Another manifestation of the back-side grindinggrinds the height of the lower power chips 3 to below the level of thehigher power chips 31 to allow the highest power chip(s) 31 to achievethe highest plane (be the tallest chip) so that the smallest chip to cap5 gap is achieved at the highest power chip 31, after the heat sink 9 isattached. Back-side grinding of the chips at the wafer or individualchip level adds significant process complexity and cost.

In FIG. 6, a planar spreader 14 is attached to a structure with groundchips so that the smallest chip to spreader 14 gap is achieved at thehighest power chip 31 for optimized cooling. A drawback to the structurein FIG. 6 is that high mechanical stress can be generated in the thermaladhesive 12 between the chip 3 and spreader 14 due to the large distancefrom neutral point (DNP) in the thermal compound 6 for the chips 3located at the extreme edges of the chip carrier 1. The high mechanicalstresses are generated due to the thermal expansion mismatch between thespreader and the underlying chip during field operation.

FIG. 7 is a variant of FIG. 6, but the structure in FIG. 7 does notrequire grinding the chip back-sides. A non-planar heat spreader 15 withsteps is used so that smallest chip to spreader 15 gap is established atthe high power chip 31. The drawback to the stepped spreader is theadded cost required to include the steps in the heat spreader 15, inaddition to the mechanical stress issue of large DNP, discussed above.

Thus, the inventive cooling structure comprises a heat dissipatingstructure (e.g., heat sink) 15 connected to the upper sides of theintegrated circuit chips 3, 31 through a thermally conductive material12, 70, where the heat dissipating structure is shaped such that thesmallest gap exists between the top of the integrated circuit chip thatproduces the most thermal energy 31 and the bottom of the heatdissipating structure 15.

In this embodiment, the heat dissipating structure has a protrusion(step) 71 positioned adjacent the integrated circuit chip that producesthe most thermal energy 31 to allow the smallest of the gaps to existbetween the top of the integrated circuit chip that produces the mostthermal energy 31 and the bottom of the heat dissipating structure 15.The thermally conductive materials 12, 70 comprise a thermallyconductive adhesive and can comprise a plurality of thermally conductivematerials having different coefficients of thermal conductivity.

In the embodiment shown in FIG. 8, differently sized heat spreaders 16and 17 are attached to the chips 3, 31 with common thermally conductiveadhesive materials 18 and 19. The thickness of the high power spreader16 (the spreader attached to the highest power chip 31) exceeds thethickness of the lower power spreaders 17 (the spreaders attached to thelower power chips 3) to ensure that the high power spreader 16 achievesthe highest plane (e.g., extends higher than (above) the lower powerspreaders 17). This ensures that when the package mates with the flatbase of a heat sink 9, the smallest thermal interface gap is achievedbetween the highest power spreader 17 (which is the tallest spreader)and the heat sink 9, allowing optimum cooling.

The spreader materials 16 and 17 can be identical or the high powerspreader 16 can have a higher thermal conductivity than the lower powerspreaders 17. Similarly, the adhesive materials 18 and 19 andgreases/solders 20, 21 on the chips can be identical, or an adhesivehaving a higher thermal conductivity 18 (when compared to adhesive 19)and grease/solder having a higher thermal conductivity 20 (when comparedto grease/solder 21) can be used to attach the high power heat spreader16. As would be understood by one ordinarily skilled in the art,different combinations of high/low thermally conductive spreaders,adhesives, greases, and solders can be used with the invention toachieve any specific desired ratios of thermal conductivity.

Thus, the embodiment shown in FIG. 8 presents a cooling structure for anintegrated circuit structure that has multiple integrated circuit chips3, 31. More specifically, the cooling structure comprises heat spreaders16, 17 and a heat dissipating structure 9 connected to the upper sidesof the heat spreaders 16, 17 through a thermally conductive material 20,21. The lower side of each of the heat spreaders 16, 17 is connected tothe top of a corresponding integrated circuit chip 3, 31. The thermallyconductive material 20, 21 is positioned in gaps between the upper sidesof the heat spreaders 16, 17 and the bottom of the heat dissipatingstructure 9, and the smallest of the gaps exists between the top of theheat spreader 16 that is connected to the integrated circuit chip 31that produces the most thermal energy, relative to the other integratedcircuit chips, and the bottom of the heat dissipating structure 9.

As shown above, the heat spreaders 16, 17 can have different thicknessesand can have different coefficients of thermal conductivity. One or morethermally conductive adhesives 18, 19 connect the heat spreaders 16, 17to the integrated circuit chips 3, 31. The thermally conductive material20, 21 can comprise a plurality of thermally conductive materials havingdifferent coefficients of thermal conductivity and can be a thermalgrease or a phase change material. As mentioned above, the integratedcircuit chips comprise at least one higher power chip 31 and at leastone lower power chip 3, wherein, during operation, the higher power chip31 generates more thermal energy than the lower power chips 3.

FIG. 9 illustrates a cap 22 attached to the chip carrier 1 that replacesthe heat spreader 16 on the high power chip 31, while heat spreaders 17are used on the lower power chips 3. The cap 22 is formed to lie in ahigher plane than the heat spreaders 17 (e.g., to have an upper surfacethat is above the upper surfaces of the heat spreaders 17) so that thecap is taller (higher) than the lower power spreaders 17. The heat sink9 would be connected to the cap 22 and heat spreaders 17 as shown inFIG. 8, and the smallest gap would exist between the cap 22 and the heatsink 9 in a similar manner to the heat spreader 16 above. One ordinarilyskilled in the art would understand that caps of different heights couldbe formed above all the chips 3, 31, with the highest power chip 31having the tallest cap. When caps are used, adhesives are not necessary.Therefore, in the embodiments with caps, the adhesive 18 between thechip 31 and cap 22 can be replaced by a soft material such as a thermalgrease or phase change material.

Thus, in the embodiment shown in FIG. 9, the cooling structure comprisesa cap 22 connected to the chip carrier 1 and to the top of theintegrated circuit chip 31 that produces the most thermal energy,relative to the other integrated circuit chips. This embodiment alsoincludes a plurality of heat spreaders 17, wherein the lower side ofeach of these heat spreaders 17 is connected to the top of acorresponding integrated circuit chip of the other integrated circuitchips 3. In this embodiment, the heat dissipating structure 9 isconnected to the upper sides of the heat spreaders 17 and the cap 22through the thermally conductive material 20, 21. The thermallyconductive material 20, 21 is positioned in gaps, wherein the gaps existbetween the tops of the heat spreaders 17 and the bottom of heatdissipating structure 9, and between the upper side of the cap 22 andthe bottom of the heat dissipating structure 9. Here, the smallest ofthe gaps exists between the upper side of the cap 22 and the bottom ofthe heat dissipating structure 9.

As shown in FIG. 9, the heat spreaders 17 have different thicknessesthan the cap 22 and the heat spreaders 17 may have different thicknessesthan each other. A movable piston 40 can also be included in the cap, asshown in FIG. 10, to minimize the thickness of the thermal interfacematerial 18 between the chip 31 and the cap 22. During manufacturing,the piston 40 can be moved into a position to minimize the gap betweenthe bottom of the piston 40 and the top of the highest power chip 31.After this, the position of the piston 40 is fixed and the top surfaceof the cap 22 and piston 40 are planarized. As mentioned above, the heatspreaders 17 may have different coefficients of thermal conductivitythan the cap 22. A thermal adhesive 19 connects the heat spreaders 17 tothe integrated circuit chips 3. This thermally conductive material 19can comprises a plurality of thermally conductive materials havingdifferent coefficients of thermal conductivity. The thermally conductivematerial 18 comprises one of a thermal grease and a phase changematerial. The heat sink 9 and thermally conductive materials 20, 21 areattached to the structures shown in FIGS. 9 and 10.

In FIG. 11, the heat sink 23 is modified to form a cavity 24 in whichthe high power spreader 16 or cap 22 is enclosed. The heat sink 23 caninterface directly to the lower power chips 3 through well-knowncompressible thermal interface materials 25. Thus, the resultantvariable gaps between the lower power spreaders and the heat sink baseare accommodated by highly compressible thermal pad or grease materialswhich have lower required thermal performance but can span gaps of theorder of millimeters.

Thus, FIG. 11 illustrates a further embodiment where the coolingstructure comprises a heat spreader 16 connected to the integratedcircuit chip that produces the most thermal energy 31 and a heatdissipating structure 23 connected to the upper side of the heatspreader 16 and to the upper sides of the other integrated circuit chips3. Thermally conductive material 20, 25 is positioned in gaps that existbetween the upper side of the heat spreader 16 and the bottom of theheat dissipating structure 23, and between the upper sides of theintegrated circuit chips 3 and the bottom of the heat dissipatingstructure 23. Again, the smallest of the gaps exists between the upperside of the heat spreader 16 and the bottom of the heat dissipatingstructure 23.

The heat dissipating structure 23 includes a recess 24 for accommodatingthe heat spreader 16. A thermal adhesive 18 connects the heat spreader16 to the integrated circuit chip that produces the most thermal energy31. The thermally conductive materials 18, 25 can comprise a pluralityof thermally conductive materials having different coefficients ofthermal conductivity. The thermally conductive material 20 comprises oneof a thermal grease and a phase change material.

While the foregoing examples use a structure that includes two lowerpower chips 3 and one higher power chip 31, the invention is equallyapplicable to structures that include many higher power chips. In such asituation, the invention can minimize the gap for all such higher powerchips or select a very limited number (e.g., one or two) chips that willreceive the most cooling. Each situation will vary depending upon thedesign involved and the cooling needs of the chips. Additionally, whilethe highest power chip is presumed to produce the most heat, if a lowerpower chip were to require the highest level of cooling in a givendesign, the invention can minimize the gap to the heat sink relating tothe chip (or other circuit element) that requires the greatest level ofcooling.

Thus, the cooling structure described in the present invention can haveindividual spreaders or caps mounted on the chips. The thickness(height) of the higher power spreader or cap is designed to exceed theheight of the lower power spreaders to ensure that the high powerspreader achieves the highest plane. This ensures that when the packagemates with the flat base of a heat sink, the smallest thermal interfacegap is achieved between the highest power spreader and the heat sink.The smallest gap will produce the greatest amount of cooling becauseless heat loss occurs when the thermal energy travels through a smalleramount of thermal gap material. Thus, by providing the smallest gapabove the highest power chip, the highest power chip (which produces themost heat) is provided with the greatest amount of cooling.

The benefits of the invention are the low-cost of the inventive coolingassembly, the concentration of highest cooling capability on the higherpower chips, and the broad adaptability to different package I/Oschemes. The invention is applicable to multi-chip packages with CGA,BOA, LGA and other package I/O schemes.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

1. A cooling structure for an integrated circuit structure havingmultiple integrated circuit chips, said cooling structure comprising: aplurality of heat spreaders having different thicknesses, wherein thelower side of each of said heat spreaders is connected to the top of acorresponding one of said integrated circuit chips; and a heatdissipating structure having a flat base connected to the upper sides ofsaid heat spreaders through a thermally conductive material, whereinsaid thermally conductive material is positioned in gaps between theupper sides of said heat spreaders and said flat base of said heatdissipating structure, and wherein the smallest of said gaps existsbetween the top of the thickest of said heat spreaders that is connectedto the integrated circuit chip that produces the most thermal energy,relative to the other integrated circuit chips, and said flat base ofsaid heat dissipating structure.
 2. The structure in claim 1, whereinsaid heat spreaders have different coefficients of thermal conductivity.3. The structure in claim 1, further comprising a thermal adhesiveconnecting said heat spreaders to said integrated circuit chips.
 4. Thestructure in claim 1, wherein said thermally conductive materialcomprises a plurality of thermally conductive materials having differentcoefficients of thermal conductivity.
 5. The structure in claim 1,wherein said thermally conductive material comprises one of a thermalgrease and a phase change material.
 6. The structure in claim 1, whereinsaid integrated circuit chips comprise at least one higher power chipand at least one lower power chip, wherein, during operation, saidhigher power chip generates more thermal energy than said lower powerchip.
 7. The structure in claim 6, wherein said smallest of said gapsallows optimum cooling of said at least one higher power chip.
 8. Acooling structure for an integrated circuit structure having multipleintegrated circuit chips, said cooling structure comprising: a pluralityof heat spreaders, having different thicknesses, wherein the lower sideof each of said heat spreaders is connected to the top of acorresponding integrated circuit chip, wherein a first chip of saidmultiple integrated circuit chips produces the most thermal energyrelative to others of said multiple integrated circuit chips, andwherein the thickest of said heat spreaders is connected to said firstchip so as to optimize cooling of said integrated circuit structure; anda heat dissipating structure having a flat base connected to the uppersides of said heat spreaders through a thermally conductive material. 9.The structure in claim 8, wherein said heat spreaders have differentcoefficients of thermal conductivity.
 10. The structure in claim 8,further comprising a thermal adhesive connecting said heat spreaders tosaid integrated circuit chips.
 11. The structure in claim 8, whereinsaid thermally conductive material comprises a plurality of thermallyconductive materials having different coefficients of thermalconductivity.
 12. The structure in claim 8, wherein said thermallyconductive material comprises one of a thermal grease and a phase changematerial.
 13. The structure in claim 8, wherein said integrated circuitchips comprise at least one higher power chip and at least one lowerpower chip, wherein, during operation, said higher power chip is saidone of said multiple integrated chips that produces the most thermalenergy.
 14. The structure in claim 8, wherein said thermally conductivematerial is positioned in gaps between the upper sides of said heatspreaders and said flat base of said heat dissipating structure, andwherein the smallest of said gaps exists between the top of saidthickest heat spreaders and said flat base.
 15. A cooling structure foran integrated circuit structure having multiple integrated circuitchips, said cooling structure comprising: a plurality of beat spreaders,having different thicknesses; wherein the lower side of each of saidheat spreaders is connected to the top of a corresponding integratedcircuit chip, wherein a first chip of said multiple integrated circuitchips produces the most thermal energy relative to others of saidmultiple integrated circuit chips, and wherein the thickest of said heatspreaders is connected to said first chip so as to optimize cooling ofsaid integrated circuit structure; and a heat dissipating structurehaving a flat base connected to the upper sides of said heat spreadersthrough a thermal grease.
 16. The structure in claim 15, wherein saidheat spreaders have different coefficients of thermal conductivity. 17.The structure in claim 15, further comprising a thermal adhesiveconnecting said heat spreaders to said integrated circuit chips.
 18. Thestructure in claim 15, wherein said integrated circuit chips comprise atleast one higher power chip and at least one lower power chip, wherein,during operation, said higher power chip is said one of said multipleintegrated chips that produces the most thermal energy.
 19. Thestructure in claim 15, wherein said thermal grease is positioned in gapsbetween the upper sides of said heat spreaders and said flat base ofsaid heat dissipating structure, and wherein the smallest of said gapsexists between the top of said thickest heat spreader and said flatbase.